PURPOSE: To obtain a residual fuel detector which can detect residual quantity even for alcohol mixed fuel correctly.

CONSTITUTION: A control unit 1 determines a correction coefficient for obtaining the quantity of pure gasoline in alcohol mixed fuel based on an alcohol mixing rate. The control unit 1 is connected with a fuel sensor 5 for detecting residual quantity in a fuel tank and processes the output therefrom to calculate quantity of residual fuel and then calculates quantity of gasoline in the residual fuel by multiplying the quantity of residual fuel by the correction coefficient. Quantity of gasoline thus calculated is indicated on a meter 3 provided on an instrument panel. Quantity of gasoline in the residual fuel is further compared with a predetermined reference value and when it drops below the reference value, an alarm lamp is lighted to notify the fact to a driver.